OK, today was my next opportunity to buy a series of tickets, and I'm still getting the error.
Same scenario: new tickets, no TTF, using a U.S.-based Mastercard. Gave it a bland no-space name.
Went to purchase 2nd ticket, received this error:
Originally Posted by Error Message
We are currently unable to complete your request due to an undefined payment error. If this is the first time you have received this message, please try again. If you have received this message more than once, please contact a Southwest Airlines Customer Representative for assistance at 1-800-I-FLY-SWA (1-800-435-9792). However, please note that some fares, offers, and functionality are only available through our website. (SW900001-C2FD-033604069)
I was able to type in the CC number again, manually, and buy the ticket.
Looks like a different error code but I don't know why I'm getting this error at all.